The broad, long-term objectives of this research are to understand the mechanisms that regulate hair cell regeneration in the chick cochlea. The basilar papilla, the auditory epithelium of the chick cochlea, is normally mitotically quiescent after embryonic development. However, in response to noise exposure or aminoglycoside administration the nonsensory supporting cells emerge from quiescence and divide to produce a pool of progenitor cells. A subset of the progenitors go on to differentiate into new hair cells. Regeneration in the chick cochlea involves the formation of neural synapses on these new cells and relatively complete physiological and behavioral recovery of auditory function. Once the mechanisms that induce supporting cells in the chick cochlea to proliferate, differentiate into mature hair cells, and establish functional connections with higher brain centers are understood, it may be possible to apply this knowledge to the amelioration of deafness in humans caused by environmental, age-related, or genetically-based hair cell loss. The four specific aims of this proposal will examine: 1) The role of growth factors retinoids, and their receptors in the regulation of supporting cell proliferation and hair cell differentiation. Two factors and their receptors have been identified that are actually present in the chick cochlea and may potentially regulate regeneration. 2) How the cell-cell and cell-matrix attachments of the cochlear epithelium control cell fate, cell function, and the ability to regenerate. Identified differences in the composition of the basilar membrane under supporting cells and hyaline cells may regulate at he cell fate and proliferative capacity of the overlying cells in the cochlea. The chick cochlea offers a unique opportunity to examine these relationships in vivo. Almost all other studies on integrins, focal adhesions, and their extracellular matrix interactions have been performed on cells and tissues in culture. 3) How aminoglycoside treatment induces the regeneration of hair cells land the synthesis of new tectorial membrane, as compared to noise-induced regeneration. An understanding of the changes following aminoglycoside treatment will provide an excellent comparative model for examining the role of tall and short hair cell contributions to functional recovery. 4) How noise and aminoglycoside damage affect the normal pattern of afferent and efferent nerve fibers and their synapses in the chick cochlea. Comparisons between control, noise damaged, and gentamicin treated ears will elucidate the changes in innervation that occur when hair cell loss involves predominantly short hair cells versus that which includes both tall and short hair cells. The differences may explain why recovery of function takes much longer after gentamicin treatment then after noise exposure. These four specific aims will be addressed by a combination of techniques including scanning and transmission electron microscopy, epifluorescent and video-enhanced DIC light microscopy, confocal laser scanning microscopy, immunocytochemistry of cochlear whole mounts, and autoradiography of sectioned cochlear tissues.